In traditional Class AB audio amplifiers, current limiting of the output signal is easily implemented and a standard feature. This feature serves as a protection feature in case of a short circuit of the amplifier output terminals, but also as a current clipping feature. The benefit of the current clipping feature is that the audio amplifier will continue playing music even in case of an impedance-dip in the transducer or at local maximum of the music material. In the audio amplification it is seldom accepted to have a latched shutdown or a temporary shutdown in case of an over-current situation. Music at all time is very important and often a requirement.
In Class D audio amplifiers, the current protection/clipping is somewhat more difficult to implement because of the switching nature of these amplifiers. It becomes increasingly difficult if cycle-by-cycle current limiting without shutdown is needed. For clocked Class D audio amplifiers, the over-current and clipping feature can be implemented using well-known current-limiting methods known from switched mode power supply design. Using this method, also known as Current Programmed Control (CPC), the cycle-by-cycle current-limiting feature enables current clipping as known from Class AB audio amplifiers.
In case of voltage-controlled self-oscillating Class D amplifiers, the standard current programmed control scheme can only be used as latched shutdown or restart after over-current detection. The cycle-by-cycle current limiting is not possible using the standard methods. Therefore, in voltage-controlled self-oscillating Class D amplifiers, the current clipping feature is not an option. For the voltage-controlled self-oscillating Class D amplifiers, the voltage loop during an over-current event will saturate and all switching action will stop. This results in holes in the audio signal and noisy restart phenomena caused by recovery from saturation.
Besides the considerations regarding using the right current limit strategy, the current detection method can also give rise to certain problems.
The most popular and inexpensive way to measure the current is by adding a sense resistor in the power path. The current limit is then reached when the voltage across the sense resistor reaches a predetermined value. The most significant drawback of this method is the power loss associated with this method. Adding a sense resistor in the power path will contribute to a larger switching loop with the associated drawbacks of larger EMI pollution. For small power amplifiers, the losses and the added power path loop are usually manageable but for higher power levels, the power dissipation and the increased switching loop become a severe problem.
To manage the higher power levels, current sense transformers can be used. This approach will take care of the power loss problem and reduce the size of the switching loop, but the cost is increased. Furthermore, it is not in all applications that this approach is manageable because of saturation phenomena in the current transformer.
A hall-sensor can also be used to measure the current and this approach will solve the saturation problem of the current transformer. The disadvantage of this method is the cost, particularly because of the high demand on the hall-sensor in terms of bandwidth.
In general all of the above-mentioned current sense methods interrupt the power path which in this switching environment can give rise to EMI problems Also the location of the current sense object can give rise to trouble—especially if sense resistors are used. Typically the best location for the sense device is also the most difficult location for the detection circuitry to interface to; hence the use of expensive operational amplifiers is often required.